Japanese patent application Ser. No. 87053/1972, filed Aug. 29, 1972, and published May 24, 1973, under No. 35357/1973, describes a method of manufacturing materials for printed circuits, wherein an ultra-thin layer of copper is deposited upon a suitable temporary carrier such as a substrate of aluminum, zinc or steel.
The copper foil, which may have a thickness of 17 microns and less, is then laminated with an insulating support such as a glass fiber reinforced epoxy resin. Thereafter, the temporary carrier is removed, either by mechanically peeling the same off, or chemically, by etching it off, leaving a copper foil surface on an insulating support. The copper foil may then be etched or otherwise treated to produce an electrical circuit.
The use of ultra-thin copper foil in the manufacture of printed circuit boards provides some distinct advantages over the use of conventional "one ounce" copper foil, i.e. copper foil weighing one ounce per square foot, and measuring about 1.4 mils thick. For example, since the ultra-thin foil is on the order of only 1/6 to 1/7 as thick as one ounce copper foil, there is an initial and substantial saving in the quantity of copper employed. Moreover, since less copper is removed during etching, copper recovery costs are similarly reduced, or alternatively, copper losses are reduced if no such recovery is effected.
Further, the problem of undercutting of retained copper by the etchant during printed circuit formation, which is a significant problem when using one ounce copper foils and thicker, is minimized with the use of ultra-thin copper foils. This is due to the fact that the severity of undercutting decreases as the thickness of the copper foil decreases.
The above mentioned Japanese application indicates that some difficulty has been experienced in obtaining good adhesion between an aluminum temporary substrate and electrocathodically deposited copper. The disclosure goes on to state that in order to obtain good adhesion, it is essential to use a cyanide electroplating bath, and that even when such a bath is employed, the plated product tends to deteriorate with time. The Japanese disclosure also makes note of the hazardous nature of cyanide plating solutions.
As a solution to these problems, the Japanese disclosure states that the aluminum substrate may be coated with a thin layer of zinc or tin prior to depositing an electroplate of copper.
It is also known, in accordance with the prior art, to apply a thin layer of zinc, indium, or brass to a copper foil prior to laminating the foil to a suitable support to prevent migration of nodules of copper or copper oxide on the surface of the foil to the interior of the support. As described in British Pat. No. 1,293,801, the application of such a barrier layer solves the migration problem with no loss of peel strength.
Finally, it is known according to U.S. Pat. No. 2,856,333 to etch aluminum alloys with HCl fumes to provide a surface to which a copper electroplate will strongly bond.